Absorption spectra modification in poly(3-hexylthiophene):methanofullerene blend thin filmsIntroductionExperimentalResults and discussionConclusionsAcknowledgmentsReferencesAbsorption spectra modification inpoly(3-hexylthiophene):methanofullerene blend thin filmsVishal Shrotriya, Jianyong Ouyang, Ricky J. Tseng, Gang Li, Yang Yang*Department of Materials Science and Engineering, University of California, Los Angeles, CA 90095, USAReceived 15 April 2005; in final form 8 June 2005AbstractThe absorption spectra and the photovoltaic effect in thin films consisting of a blend of p-type poly(3-hexylthiophene) and n-typeacceptor [6,6]-phenyl C60 butyric acid methyl ester have been studied and a decrease in inter-band absorption in the wavelengthrange of 450–600 nm is observed. This absorption quenching is attributed to the disordering of the poly(3-hexylthiophene) chainsand a charge transfer between poly(3-hexylthiophene) and [6,6]-phenyl C60 butyric acid methyl ester, as evidenced by FTIR and X-ray photoelectron spectra. Finally, photovoltaic cells were fabricated utilizing the blend as the active layer and the device charac-teristics were studied.Ó 2005 Elsevier B.V. All rights reserved.1. IntroductionPolymer based photovoltaic devices, incorporating3-D interpenetrating network of conjugated polymersand C60derivatives, have shown potential for applica-tion as renewable source of energy [1] . There have beenseveral efforts in last few years to improve the efficienciesof these devices and reach the levels where they can beput into practical applications [2–5]. To furt her improvepower conversion efficiency of the photovoltaic cells, itis important to use high-mobility polymers as the activematerial. In terms of literature, poly(3-hexylthiophene)(P3HT) has the highest charge carrier mobility amongthe conjugated polymers and hole mobility as high as0.1 cm2V1s1has been reported [6]. Moreover,P3HT has a band gap of 1.9–2.0 eV, which matches wellwith the strongest sun light [7]. These merits renderP3HT good candidate for the polymer in polymer photo-voltaic cells [8–10]. Several laboratories have reportedhigh-performance photovoltaic cells using P3H T as thedonor and [6,6]-phenyl C60 butyric acid methyl ester(PCBM) as the acceptor. A power efficiency of 3.5%has already been reached for the device utilizing a blendof P3HT and PCBM (in 1:2 wt. ratio) after a treatmentof thermal annealing and electrical con ditioning of thedevices [9] . Recently, it has been shown that for photo-voltaic devices utilizi ng P3HT:PCBM blends, the donorto acceptor ratio of 1:1 by weight gives the best deviceperformance [11,12], with power conversion efficiencyof about 3% being reported [11]. To further enhancethe device performance, a study on the interactionbetween P3HT and PCBM is very important. Weobserved that the absorption spectra of thin films ob-tained by spin-coating a blend of P3HT:PCBM fromsolution showed a significant blue-shift when theamount of PCBM is 67 wt.% or more (i.e., at 1:2 wt.ratio of P3HT:PCBM), resulting in reduced red partabsorption. Chirvase et al. [12] have reported similartrend in the absorption spectra modification and have0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.cplett.2005.06.027*Corresponding author. Fax: +1 310 825 3665.E-mail address: [email protected] (Y. Yang).www.elsevier.com/locate/cplettChemical Physics Letters 411 (2005) 138–143ARTICLE IN PRESSattributed this change to destruction of ordering in theP3HT chains in the presence of PCBM. From our exper-iments, we demonstrate that the absorption spectramodification in P3HT:PCBM composite films is due totwo possible interactions: first, the destruction of order-ing of P3HT chains, as suggested earlier by Chirvase etal. [12], and second, due to a non-photoinduced chargetransfer between P3HT and PCBM.2. ExperimentalA typical polymer photovoltaic device in this studyconsisted of a layer of polymer thin film sandwichedbetween a transparent anode (indium tin-oxide, ITO)and a metal cathode. The active polymeric material isan admixture of P3HT , a p-type polymer, and PCBM,an n-type acceptor. For fabricating the devices, theITO glass substrates were cleaned by ultrasonic treat-ment in detergent, de-ionized water, acetone and isopro-pyl alcohol sequentially. The ITO surface was thenmodified by spin-coating a thin layer (about 30 nm) ofPEDOT:PSS (BaytronÒP VP Al 4083) from water. Thiswas followed by thermal treatment of the substrates at120 °C for 2 h. Then, solut ions of P3HT blended withdifferent wt. ratios of PCBM, were spin-coated from1,2-dichlorobenzene (DCB) on the prepared anodes.The thickness of the polymer films was controlled bychanging and optimizing the spinning speeds. Finally,the cathode, consisting of 30 nm of Ca and 80 nm ofAl layers, was thermally deposited on top of the poly-mer film under vacuum of 106Torr. The active areaof the device was around 0.11 cm2. For obtainingabsorption spectra, thin films of P3HT:PCBM blendwere spun-cast from solution in DCB onto quartz sub-strates and the spectra were obtained from Varian Cary50 UV–Visible spectrophotometer. Similarly for obtain-ing infrared (IR) spectr a the films were spun-cast onpotassium bromide (KBr) palates and the measurementswere done using Nicolet Avatar 360 FTIR Spectrome-ter. The X-ray Photoelectron Spectroscopy (XPS) datawas obtained using monochromatic Al Ka radiation(1486.6 eV, 300 W) source with analyzer pass energyof 50 eV. The base pressure of analysis chamber waslower than 109Torr. The film samples were preparedby spin-coating thin polymer layers on Au-plated Sisubstrates mounted on stainless steel holder. Fromall the spectra, a linear constant background wassubtracted and the peak fitting was performed with asymmetric Gauss–Lorentz function. The spin–orbitsplittings were set at 1.20 eV for the S 2p signals. Theresolution of the measurement is about 0.1 eV. The cur-rent–voltage (I–V) curves were measured by a Keithley2400 source-measure unit. The photocurre nt was mea-sured under illumina tion supplied by a ThermalOriel150 W solar simulator (AM1.5G conditions). The thick-nesses of the various films were measured using Dekta kprofilometer. All devices were fabricated and tested inoxygen and moisture free nitrogen ambient inside theglove-box.3. Results and discussionFig. 1a shows the UV–Vis absorption spectra mea-sured for P3HT from film and solution